Stem cell and progenitor cell isolation and characterization are the subjects of intense research because of the great potential of such cells. The totipotent stem cells, which have the capacity to become any type of cell in a human body, give rise to progenitor cells more differentiated than the totipotent cell. One of these types of progenitor cells is the pre-determined pancreatic epithelial progenitor cell. The pancreatic epithelial progenitor cells have the ability to become different types of pancreatic epithelial cells. The different types of pancreatic epithelial cells include acinar cells, islet cells, and ductal cells. Acinar cells are generally found near the head of the pancreas and contain zymogen granules which are readily visible by electron microscopy. Acinar cells perform exocrine functions by discharging alkaline digestive juices into the small intestine. Approximately 1500 mL of pancreatic juices are secreted per day and include enzymes needed to break up lipids and proteins. Ganong, William F. Review of Medical Physiology, Chapter 26 “Regulation of Gastrointestinal Function”, Fifteenth Edition, Appleton and Lange (1991). There are four types of islet cells, also known as islets of Langerhans, islet-α, isle-β, islet-δ, and islet-PP. Islet-αcells secrete glucagon which promotes gluconeogenesis, i.e. breakdown of energy reserves to generate more circulating glucose. Islet-β cells secrete insulin which promotes storage of circulating glucose into accessible energy resources. In type I diabetes mellitus, otherwise known as juvenile diabetes, it is thought that autoimmune attacks on islet-β cells cause defective islet-β cell function, thereby causing a lack of insulin to reduce the levels of circulating glucose. Islet-δ cells secrete somatostatin which regulates the secretion of glucagon and insulin. The fourth islet cell type islet-PP (pancreatic polypeptide) does not yet have a known function within the pancreas. Another type of sub-pancreatic cell is the ductal cell. These cells line the ducts that connect different parts of the pancreas.
Isolation of pancreatic epithelial progenitor cells, as with other types of progenitor cells, is difficult because of the ephemeral nature of progenitor cells. Manipulation of progenitor cells required for isolation may disturb the fragile progenitor status of these cells and may cause them to differentiate. Contact with growth factors or substrates may also induce a pancreatic progenitor cell to begin differentiating into exocrine or endocrine cells. Research in the area of pancreatic cells has resulted in the establishment of several pancreatic epithelial cell lines derived from rats. Stephan, J. et. al. Endocrinology 140:5841–5854, (1999). Other research includes the isolation of human adult pancreatic cells and the induction of these pancreatic cells to proliferate into islet-β-like structures with hepatocyte growth factor/scatter factor (HGF/SF). Jeffrey et. al. U.S. Pat. No. 5,888,705. Other research work involves inducing growth of islet cells from adult pancreatic cells by culturing first in serum-containing, low-glucose medium and then switching to medium with higher serum and glucose content. WO 9715310. Still other research in the area of pancreatic progenitor cells includes isolating progenitor cells from pre-diabetic adults and culturing in a serum-containing, pre-defined media that promotes the growth of functional islet cells. U.S. Pat. No. 5,834,308. However, all of these “progenitor” cells give rise only to islet cells. Pancreatic cells of the aforementioned research do not have the capacity to differentiate into both endocrine and exocrine cell types. It seems likely that the pancreatic cells of the aforementioned research are further committed down the differentiation pathway of pancreatic progenitor cells and therefore are different types of pancreatic cells than the human pancreatic progenitor cells of this invention. Furthermore, culturing conditions used in the aforementioned research wherein serum is used to supplement media may have adverse consequences. Serum, the fluid portion of blood after blood has been allowed to clot, contains many biomolecules such as albumin and α, β, -globulins. In vivo, cells are not normally exposed to an equivalent of serum unless tissue injury was involved. Therefore, culturing pancreatic cells in serum may not accurately reflect the physiological parameters within which pancreatic cells exist in vivo.
The ideal population of pancreatic progenitor cells should be able to differentiate into exocrine (i.e. acinar) cells, endocrine (i.e. islet-α, islet-β, islet-δ, and islet-PP) cells as well as ductal cells. Such a population of pancreatic progenitor cells may be useful in clinical settings, for example to treat certain types of diabetes or to treat functionally defective pancreatic cells by transplantation of pancreatic progenitor cells that can differentiate into functional pancreatic cells. Accordingly, there is a need for a population of pancreatic progenitor cells and methods of isolating and culturing the pancreatic progenitor cells such that the differentiation potential of the pancreatic progenitor cells is retained while permitting proliferation and avoiding senescence of these cells. The pancreatic progenitor cells and methods of isolating and culturing these pancreatic progenitor cells disclosed herein satisfies these needs and also provides related advantages.